In existing land mobile radio systems, the available bandwidth is divided by frequency into discrete channels. This technique is referred to as Frequency Division Multiplexing (FDM). The purpose of this channelization is to allow many users to simultaneously use available spectrum without interference. As an example, FIG. 1 illustrates the situation in the 800 land mobile band.
In the 800 MHz band, each radio channel actually comprises two allocations: a 25 KHz-wide allocation between 806-821 MHz for Base-to-Mobile transmission, and a 25 KHz-wide allocation between 851-866 MHz for Mobile-to-Base transmission. The spacing between the two 25 KHz allocations is always 45 MHz, this spacing known in the art as "T/R spacing". All users must control their modulation in conformance with FCC regulations such that their emissions remain within their assigned 25 KHz allocated bandwidth. It will be apparent that other land mobile bands may differ in allocated bandwidth values and T/R spacing, but are otherwise similar.
In certain bands such as, for example, the 800 MHz band, groups of channels may be organized into the common trunked radio system arrangement. The number of channels in such a system is typically from 5 to 20 channels; these channels are usually non-contiguous. In a typical trunking system such as, for example, the Motorola "Privacy Plus" system, one of the groups of channels is dedicated to control, and a central controller arbitrates the use of the remaining channels. Mobile users communicate with the central controller via the control channel to request and thereby receive allocation for one of the system's radio channels for their temporary use.
This FDM approach outlined above has proved satisfactory for voice communications, since voice signals typically occupy a predictable or constant bandwidth. On the other hand, often this approach is very unsatisfactory for data communication applications. One reason for this is that data communications typically are "bursty" and non-uniform in their bandwidth requirements. For instance, a data application may require very little bandwidth at some points in the session, but very great bandwidth at other points in the session. In the FDM approach, the maximum available data throughput is restricted by the fixed channel allocation bandwidth-and this is often too low to allow satisfactory operation of data applications.
If we now consider a trunked radio system, the bandwidth limit imposed by FDM appears even more unsatisfactory. This is because once a user has obtained a channel allocation from the central controller, he is limited by the data throughput of that assigned channel. One irony of this unfortunate situation is that other channels in the trunked system might be unused and therefore available at the very time a user's throughput becomes blocked since he is limited to only one channel.